This invention relates to apparatus and methods for electroplating metal layers onto substrates. More specifically, it relates to apparatus including anodes of a specified structure and membrane separators defining anode chambers.
Damascene processing is the principal method for forming copper metal lines on integrated circuits. In a typical Damascene process, copper lines together with blind vias (which comprise Damascene or dual Damascene interconnections) are created by the following sequence: (I) form trench pattern in dielectric on wafer face using etch resistant photoresist, (2) etch features (trenches), (3) remove resist, (4) form via pattern in dielectric on wafer face using etch resistant photoresist, (5) etch vias, (6) remove resist, (7) deposit PVD Tantalum barrier and copper seed, (8) electroplate copper to fill etched features, and (9) polish copper off the wafer face leaving copper filled interconnect circuitry.
Since an integrated circuit is composed of large numbers of very small features (having dimensions approaching 0.1 micrometers), it is extremely important to produce these features using a process that generates very few defects in the features. While defects can arise at any step in the process flow, one area that requires special attention is operation 8 above (electroplating). During electroplating, any condition that alters the local deposition characteristics can give rise to a defect in the final line structure. Some defects are manifest as protrusions or irregular growths of copper, and others as pits or regions of reduced local growth. In many cases, the defect or irregularity remains after the final CMP process.
One source of abnormal growth is the arrival and adherence of particulate material on the wafer surface. In some cases, a particle may arrive at the wafer surface and act to inhibit further copper growth by blocking the sites at which copper reduction will normally take place. This will result in pit formation. In another case, a particle may arrive at the wafer surface and provide an additional conductive surface on which copper growth can take place. This will result in a protruding defect, commonly called a xe2x80x9cnodulexe2x80x9d in electroplating parlance. In still another case, the particle may be coated with or composed of chemicals, which strongly alter the copper reduction process rate where they adsorb on the wafer. This can result in either a pit or a nodule depending on the nature of the chemicals in the particle reaching the wafer surface.
Electroplating baths are normally filtered to remove particles that could reach the wafer surface. Further, the baths are usually kept free, to the extent possible, of materials which could add particles to the plating solution. One material that typically cannot be avoided is a metal source anode. The metal source for most electroplating operations is an anode which dissolves to supply metal ions to the electroplating solution as ions are removed during plating. In copper electroplating, the dissolution of pure copper anodes can often result in the release of fine copper particles into the plating solution. Although these particles may eventually dissolve, they can cause the defects described above. A further problem with copper anodes arises from their propensity to decompose certain organic additives present in the copper electroplating baths. It has been found that phosphorus"" can be added to the copper anode to mitigate both of these problems. When phosphorus is present, the anode forms an adherent thick black film as it dissolves. This film prevents dissolution of small copper particles into the bulk plating solution and reduces the decomposition of the additives in the plating bath.
The black film itself is, however, a poorly structured material and can be disrupted to form a dispersion of small particles. These particles can impact growth on the wafer surface as described above unless they are prevented from reaching the vicinity of the wafer. To reduce this problem, phosphorized copper anodes may be contained in polypropylene filter bags that prevent some particles from escaping into the bulk of the solution. Unfortunately, these filters bags typically do not prevent sub-micron particles from reaching the bulk plating solution except to the extent that they reduce electrolyte flow between the anode and the bulk of the plating bath. Restricted flow at the anode causes the copper ions generated at the anode to concentrate near the anode and eventually passivate the anode.
During typical use of a phosphorized anode the defect level on the wafer surface is initially very low regardless of the particular anode material or the flow characteristics. This correlates to plating before a film, which can become dislodged in solution has formed. As plating continues the film on the anodes begins to build up as a function of current flow and to dissolve at a rate dependent on its chemical stability. The film begins to dislodge and defects begin to from on the wafer surface at a level depending on the nature of the film and the flow into the anode chamber and between the anode chamber and the wafer surface.
What is needed therefore is improved electroplating technology that reduces particulate generation at the anode and/or constrains particles generated at the anode.
The present invention addresses the above difficulties by improving anode film integrity and at the same time employing apparatus that separates the anode from the cathode and prevents most particles generated at the anode from passing to the cathode. The separation is preferably accomplished by interposing a microporous chemical transport barrier (sometimes referred to as a xe2x80x9cmembrane separatorxe2x80x9d) between the anode and cathode. The transport barrier should limit the chemical transport (via diffusion and/or convection) of most species but allow migration of anion and cation species (and hence passage of current) during application of electric fields associated with electroplating. The anode film integrity is improved by providing a phosphorized copper anode having relatively large grain sizes. The relatively few particles that are generated at the phosphorized copper anode are prevented from passing into the cathode (wafer) chamber area and thereby causing defects in the part.
One aspect of this invention provides an apparatus for electroplating copper onto a substrate. The apparatus may be characterized by the following features: (a) a cathode electrical connection that can connect to the substrate and apply a potential allowing the substrate to become a cathode; (b) a copper anode having an average grain size of at least about 50 micrometers and comprising phosphorus in a concentration of at least about 200 ppm; and (c) a membrane separator (sometimes xe2x80x9ctransport barrierxe2x80x9d herein) defining an anode chamber. The porous membrane separator enables migration of ionic species, including metal ions, across the transport barrier while substantially preventing particles larger than about 0.05 micrometers from passing. The ionic species are driven across the barrier by migration (movement in response to the imposed electric field) but the neutral particles do not transverse the transport barrier.
In some embodiments, the membrane separator can maintain different chemical compositions for the anolyte and the catholyte. Preferably it has an average pore size of at most about 0.1 micrometers (more preferably between about 0.06 and about 0.09 micrometers.
In certain embodiments, the copper anode has an average grain size of at least about 200 micrometer, more preferably between about 200 and about 1000 micrometers, and most preferably between about 500 and about 1000 micrometers. In some embodiments, the copper anode has a phosphorous content of at least about 400 ppm, more preferably between about 200 and about 1000 ppm, and most preferably between about 450 and about 600 ppm.
The apparatus may include a reservoir for electrolyte and a flow system, coupled to the reservoir, for circulating electrolyte through the anode chamber during electroplating. To further reduce defects, the flow system may include a filter for filtering particles from the electrolyte.
In a specific embodiment, the flow system can provide electrolyte flow through the anode chamber at a flow rate of between about 100 and 2000 milliliters per minute. Further, the apparatus may provide electrolyte flow between the anode and cathode chambers (through the membrane separator) at a rate of between 100 milliliters per minute and 1 liter per minute when the flow rate in the flow system is about 2000 milliliters per minute.
Another aspect of the invention provides a method of electroplating copper onto a substrate. The method may be characterized by the following sequence: (a) providing a copper anode in an anode chamber separated from a cathode chamber by a membrane separator; and (b) applying a potential difference between the substrate and the anode to allow the substrate to become a cathode and plate the copper onto the substrate. The membrane separator enables migration of ionic species, including ions of the metal, across the membrane while substantially preventing passage of particles larger than about 0.05 micrometers. As with the apparatus described above, the copper anode preferably has an average grain size of at least about 50 micrometers and comprises phosphorus in a concentration of at least about 200 ppm.
These and other features and advantages of the present invention will be described below with reference to the associated drawings.